Anti corrosive pigments based on alkaline earth hydrogen phosphates

ABSTRACT

The invention shows anticorrosive pigments on the basis of alkaline earth hydrogen phosphates, which are distinguished in that they contain from 20 mole-% to 60 mole-% of alkaline earth carbonate, the balance being alkaline earth hydrogen phosphate of the particular like alkaline earth metal, as well as a process of producing them in that alkaline earth carbonate and phosphoric acid are in per se known manner reacted with one another at the desired molar ratio, as well as the use of the anticorrosive pigments for corrosion protection of aluminum substrates, particularly for preventing filiform corrosion in aircraft coatings.

This application is a continuation of application Ser. No. 07/359,742,filed Jun. 8, 1989 now abandoned.

Strontium chromate-containing primers are of great significance inpractice in the aircraft industry in conjunction with the production ofcoatings serving for the protection of metals, in particular ofpre-treated aluminum alloys. The best coating systems for passengeraircraft have for quite some time been made to consist of a chemicalconversion layer, viz. a chromate-, phosphate-protective layer, which isproduced on Al-surfaces by the dipping, wipe-on, spraying or rollerapplication processes as anticorrosive, and which is known e.g. underthe designation Alodine 1200, or of an acid wash primer, and thereafteran epoxyamine or polyamide-corrosion protection coating or apolyurethane (PUR) corrosion protection layer with a high-glossaliphatic PUR-topcoat. By using strontium chromate in the corrosionprotection coating filiform corrosion is prevented. Filiform corrosionis observed only on pre-treated substrates, such as Fe, Zn, Mg, steel,Ni, Cr and, in particular, Al. A (star-shaped) loss in adhesion goingout from a damaged location in the lacquer film occurs--referably atvalues of humidity of air of from 65-95% and temperatures ofapproximately 30° C.--between the chemically pre-treated substrate andthe coating, which results in the coating chipping off.

There is, for instance, also known from DE-PS 74 77 80 a group ofanticorrosive pigments, in the case of which a compound mixture, whichcontains as the cation at least two of the metals calcium, strontium,barium, magnesium, zinc and as the anion chromate ion and yet at leastone anion selected from the carbonic, phosphoric, silicic, hydrofluoricor boric acids, is deposited onto inactive fillers. In that way, achromate always is present, with best results likewise being obtainedwith strontium chromate.

However, strontium chromate belongs to the anticorrosive pigments which,as zinc chromate, are hazardous from the physiologic standpoint onaccount of their cancerogenic activity and should, if possible, nolonger be utilized.

It has been attempted to use, by way of substitutes, zinc phosphate andmodified zinc phosphates, such as shown in EU-A no. 0054266 and 0054267,which are for environmental protection reasons being employedincreasingly in lieu of zinc chromate. These zinc phosphates do not,however, lead to any change in regard to filiform corrosion. Strontiumhydrogen phosphates have also been tested, but they did not provide theprotective effect which may be determined in the case of strontiumchromate.

EU-PS 0011223 shows a corrosion inhibiting pigment and a paint on thebasis of phosphate, consisting of a mixture of from 3 to 97% by wt. ofMgHPO₄ ·3H₂ O, the balance being CaHPO₄ ·2H₂ O. The pigment isespecially suited for the protection of iron and iron alloys and may beapplied as a paint or a paste. The pigment may be exchanged up to 95% bywt. by pigment extenders, for instance barytas, talc, alkaline earthcarbonates or wollastonite, and the pigment is to be present in thedried protective layer in a pigment volume concentration of from 10 to60% by volume.

These combination-type pigments of magnesium and calcium hydrogenphosphate are, however, not very well suited for aluminum or mixedsubstrates, which do not consist exclusively of iron and iron alloys.

There still exists, therefore, a need for an anticorrosive coating formetals, in particular aluminum, which is non-toxic and yet comparablewith the known strontium chromate as concerns its resistance to filiformcorrosion.

It has now been found that carbonate-containing alkaline earth hydrogenphosphates, in particular magnesium and strontium hydrogen phosphates,exhibit better stability values to filiform corrosion than other metalphosphates, also than, in particular, magnesium hydrogen phosphate andstrontium hydrogen phosphate without any carbonate component of the samealkaline earth metal. The complex effect of these carbonate-containingalkaline earth phosphates is yet further improved by additions offluorosilicates and/or fluoroborates and/or alkali or alkaline earthfluorides, respectively. In that respect, heavy metals, but also groupsII and III of the periodic table, hence alkaline earth metals and earthmetals, and here especially Al, come into consideration as the metalproportion of the fluorosilicates and fluoroborates.

At least 40 mole-% of alkaline earth carbonate in the complex arerequired so as to provide for adequate anticorrosive action; 50 mole-%of alkaline earth carbonate, with the balance being alkaline earthhydrogen phosphate, are still better. There should not be exceeded acarbonate proportion of 80%.

The additions of fluorosilicates or fluoroborates and/or alkali oralkaline earth fluorides, respectively, are comparatively low and amountto about 0.05 to 3%, in particular to 0.1 to 1%, an amount of from 0.1to 0.5% frequently providing very good results. A few routine tests mayserve to try and find out what is the right thing in regard to this aswell as also in regard to the ratio between alkaline earth hydrogenphosphate and alkaline earth carbonate.

The following Examples will elucidate the invention.

PRODUCTION EXAMPLE

By the reaction of magnesium carbonate in an aqueous, 10-40% suspensionwith technical phosphoric acid at a molar ratio of 2:1 there is obtainedmagnesium hydrogen phosphate with a carbonate proportion of 50 mole-%.The reaction may be carried out at temperatures between room temperatureand heat of boiling of the mixture, but it is carried out preferably, inthe case of an amount of a few kilograms, at temperatures of from 30° to50° C. for a period of time of 30 minutes, with the period of time, butalso the temperature, depending upon the converted amount. In the caseof a production charge of about 5 t, for instance, the temperatureincreases on account of the resultant heat of reaction to about 90° C.,and the reaction lasts for about 3 hours. Hence, the duration of thereaction and the reaction temperature are the longer and higher, thelarger the amount used is. The increase in temperature also has theadvantageous effect that the time of reaction can be kept withinreasonable limits. The precipitated pigments are filtered off incustomary manner, manner, are dried at 105° C. and ground.

Mixed pigments of strontium carbonate with phosphoric acid were producedin the same manner.

By varying the proportions of carbonate and phosphoric acid there wereproduced mixed pigments with 0.7 mol strontium hydrogen phosphate and0.3 mol strontium carbonate and 0.4 mol strontium hydrogen phosphate and0.6 mol strontium carbonate.

To this last-mentioned pigment there were still added in one instance0.3% lead fluoroborate, in one instance 0.3% copper fluorosilicate, inone intance 0.1% potassium fluoride and in one instance 0.3% magnesiumfluorosilicate.

These pigments, which are designated as pigments 1 to 5, were compared,according to the Table shown below, in lacquer coating compositions ofthe kind customarily used in regard to the coating of aircraft, withstrontium chromate, zinc phosphate, magnesium hydrogen phosphate,strontium hydrogen phosphate in that they were applied as customaryepoxy-polyamide-based 2-K-coating for aircraft onto aluminum sheets andthen tested.

TEST

The mentioned anticorrosive pigments were tested by comparison withstrontium chromate and zinc phosphate in the base coats of coatingbuild-ups on aluminum substrates.

1. 1000 hours salt spray test DIN 50021

2. 24 hours storage in water at room temperature

3. 500 hours storage at 85% rel. humidity of the air and 25° C.,previous one-hour initiation of the scratched metal sheets in conc.HCl-vapor.

COATING SYSTEM

    ______________________________________                                        Substrate:    AL 2024 (ASTM D 1733-65) QQ-A-                                                250/4, T 3 temper                                               Conversion layer:                                                                           Yellow chromating (Alodine 1200)                                Base coating: Epoxide-polyamide resin base (for                                             formulations, see Table 2)                                                    Dry layer thickness 20 microns                                  Top coating:  Polyurethane resin base                                                       Dry layer thickness 45 microns.                                 ______________________________________                                    

RESULT

On salt spray testing in the case of water-storage, results wereobtained with the carbonate-containing anticorrosive pigments, whichresults may be said to be comparable in quality to the results obtainedwith strontium chromate.

The most important criterion in regard to the evaluation of the newanticorrosive pigments is the test concerning filiform resistance.Evaluation in that respect takes place by the measurement of the averageinfiltration in millimeters at the section.

The test results in regard to filiform resistance are shown in Table 1below.

                  TABLE 1                                                         ______________________________________                                        Comparison of filiform corrosion in millimeters                               after storage for 500 hours at 85% rel. humidity and 25° C.                               Addition  mm                                               ______________________________________                                        Strontium chromate               1                                            Zinc phosphate                   6                                            [MgHPO.sub.4 ].xH.sub.2 O        4                                            (Pigment 1)                                                                   [0.5 MgHPO.sub.4.0.5 MgCO.sub.3 ].xH.sub.2 O                                                                   2-3                                          (Pigment 2)                                                                   [SrHPO.sub.4 ].xH.sub.2 O        4                                            (Pigment 3)                                                                   [0.7 SrHPO.sub.4.0.3 SrCO.sub.3 ].xH.sub.2 O                                                                   2-3                                          (Pigment 4)                                                                   [0.4 SrHPO.sub.4.0.6 SrCO.sub.3 ].xH.sub.2 O                                                                   2-3                                          [0.4 SrHPO.sub.4.0.6 SrCO.sub.3 ].xH.sub.2 O                                                       0.3% Pb(BF.sub.4).sub.2                                                                   2                                            (Pigment 5)                                                                   [0.4 SrHPO.sub.4.0.6 SrCO.sub.3 ].xH.sub.2 O                                                       0.3% CuSiF.sub.6                                                                          2                                            [0.4 SrHPO.sub.4.0.6 SrCO.sub.3 ].xH.sub.2 O                                                       0.1% KF     2                                            [0.4 SrHPO.sub.4.0.6 SrCO.sub.3 ].xH.sub.2 O                                                       0.3% MgSiF.sub.6                                                                          2                                            ______________________________________                                    

                                      TABLE 2                                     __________________________________________________________________________    2-K-Coatings on epoxy-polyamide basis for aircraft                            Type I                  1   2   3   4   5   6   7                             __________________________________________________________________________    Component A                                                                   Epoxide resin, 75%, in toluene (molecular wt.: 900,                                                   21.6                                                                              20.4                                                                              22.3                                                                              21.8                                                                              21.8                                                                              20.9                                                                              20.9                          EP-equivalent wt. 2000-2200)                                                  n-butyl acetate         1.8 1.7 1.8 1.8 1.8 1.8 1.8                           methyl ethyl ketone     1.8 1.7 1.9 1.8 1.8 1.7 1.7                           Anti-settling agent     1.1 1.0 1.2 1.1 1.1 1.0 1.0                           n-butyl acetate         5.7 5.6 5.8 5.8 5.8 5.5 5.5                           methyl ethyl ketone     5.6 5.6 5.8 5.8 5.8 5.5 5.5                           strontium chromate      11.6                                                                              --  --  --  --  --  --                            zinc phosphate          --  13.0                                                                              --  --  --  --  --                            Pigment 1               --  --  11.1                                                                              --  --  --  --                            Pigment 2               --  --  --  11.4                                                                              --  --  --                            Pigment 3               --  --  --  --  11.4                                                                              --  --                            Pigment 4               --  --  --  --  --  12.7                                                                              --                            Pigment 5               --  --  --  --  --  --  12.7                          Rutil-titanium dioxide  2.3 2.4 2.1 2.2 2.2 2.4 2.4                           Kieselguhr              2.9 2.8 2.7 2.7 2.7 2.9 2.9                           Magnesium silicate      5.6 5.8 5.3 5.5 5.5 5.6 5.6                           Component B                                                                   Polyamide-curing agent, 70%, in xylene                                                                11.5                                                                              10.9                                                                              12.0                                                                              11.6                                                                              11.6                                                                              12.6                                                                              12.6                          (H-equivalent wt. 240)                                                        Ethylene glycol         15.5                                                                              16.1                                                                              15.0                                                                              15.5                                                                              15.5                                                                              14.4                                                                              14.4                          Toluene                 6.5 6.5 6.5 6.5 6.5 6.5 6.5                           Methyl ethyl ketone     6.5 6.5 6.5 6.5 6.5 6.5 6.5                                                   100.0                                                                             100.0                                                                             100.0                                                                             100.0                                                                             100.0                                                                             100.0                                                                             100.0                         PVK in %                24.5                                                                              26.0                                                                              23.0                                                                              24.0                                                                              24.0                                                                              23.5                                                                              23.5                          __________________________________________________________________________     Components A and B are, before processing, mixed according to volume 1:1.

I claim:
 1. An anticorrosive alkaline earth hydrogen phosphate basedpigment comprising from 20 mole-% to 60 mole-% alkaline earth carbonateand from 40 to 80 mole -% of alkaline earth hydrogen phosphate of thesame alkaline earth metal and an additive selected from the groupconsisting of fluorosilicates, fluoroborates, alkali or alkaline earthfluorides and combinations thereof, wherein said additive is present inan amount of 0.05% to 3% by weight.
 2. The anticorrosive pigment ofclaim 1 wherein said additive is present in an amount between about 0.1and 1 percent by weight based on the total weight of pigment.
 3. Theanticorrosive pigment of claim 2 wherein said additive is present in anamount between about 0.1 and 0.5 percent by weight based on the totalweight of pigment.